InGaN Semiconductor Materials And Fabrication Of Solar Water Splitting Photoelectrochemical Cell Device For Hydrogen Generation

With the growing assumption of fossil fuel and environment pollution,renewable and clean new energy source is imminently needed for the sustainable development of human society.Among numerous new energy sources,solar power is the most promising new renewable clean energy.However,so far the energy conversion efficiency of typical solar cell is still low,and it is hard to store converted solar energy.Thus,the technique of solar cell hasn't been widely accepted yet.Photoelectrochemical cell(PEC)is able to convert solar energy into chemical energy by splitting water into hydrogen and oxygen.It demonstrates a potential way to harness solar energy by hydrogen,which can be easily stored.The most fundamental key of PEC is developing suitable electrode material.Good photoelectrode material should not only be able to provide enough absorption of visible light,but also have considerable energy conversion efficiency.In addition,other factors,such as simple fabrication process,low cost and environmental impacts,are also taken into consideration.Owing to the advantages of wide and extensive direct-band gap from 0.7eV to 3.4eV,excellent physical and chemical stability,?-nitride semiconductors InGaN materials are the most favorite material system for visible spectrum region PEC applications.Therefore,the study of InGaN fabrication and opoelectronic devices is one of the hotspot in the field of semiconductor science and technology.InGaN-based opoelectronic devices exhibit great application potentials,especially for hydrogen generation via solar power PEC.In this dissertation,the study of highly efficient InGaN-based PEC starts from fabrication by means of metal-organic chemical vapor deposition(MOCVD).The growth parameters and material properties of InGaN/GaN heterojunction have been deeply studied.New type of PEC devices are fabricated and systematic investigated.The main conclusions are listed as follows:1.The growth of InGaN films has been extensively investigated.By controlling growth temperature,InGaN thin films with different indium composition have been fabricated.The impacts of growth temperature on material quality and optical properties are studied.2.The formation and effects of hexagonal defects in InGaN film are systematic investigated.It confirms most hexagonal defects originate from threading dislocations.Based on the analysis of indium fluctuation,surface morphology and optical characteristics,the ring shaped high indium composition region around hexagonal defect is discovered.The carrier trapping mechanism of defects are discussed with the help of proposed energy swamp model.3.InGaN film PECs with different indium composition are fabricated and compared.Result indicates material quality is the most important elements for device performance.And n-type doping GaN layer is adopted to enhance PEC performance.As a result,the photocurrent under 1V potential is enhanced 4 times from 0.25mA/cm2 to lmA/cm2.The incident photon conversion efficiency(IPCE)under 400nm wavelength irradiation is improved from 5%to 15%.In addition,the turn-on voltage of doping device is reduced to 0.1 V.After that,surface treatment is applied to further enhance device performance.The IPCE is increased to as much as 54%.The mechanism of surface treatment are analyzed and discussed,which provides instructions for further PEC improvements4.Two type of InGaN/GaN p-i-n PEC devices are fabricated and investigated.It is discovered although the design of gradually decreased In composition InGaN layer can provide wider absorption region of solar light,the material quality is considerably affected,which is vital for PEC device performance.Nano-PEC device is made with the help of Ni nano-ball and ICP etching.The nano-PEC show great enhancements in device performance.As a result,the turn-on voltage of nano-PEC is reduced to-0.3V,and IPCE is as much as 30%.Silvaco simulation is applied to understand the mechanism of polarization modulation.5.High quality blue and green 15-pairs InGaN/GaN MQW PECs are designed and fabricated.Both of them show good photoelectrochemical performance.Based on that structure,nano-PEC is made by nano-imprint and ICP etching techniques.Both photocurrent and IPCE of nano-PEC are significantly enhanced.And the turn-on voltage of nano-PEC is detected to be about-1V.Through Silvaco simulation,the effect of strain relaxation and polarization modulation by nano-structure is deeply discussed.SiO2/SiNx Distributed Bragg Reflector(DBR)structure is adopted at the backside of sapphire in order to compensate the absorption lose of nano-sturcture.Through FDTD simulation,we find the light intensity in nanorod with DBR structure is 3 times higher than that without DBR.As a result,the photoelectrochemical performance of nano-PEC with DBR is further enhanced by 1.5 times.The IPCE of such PEC device is as much as 60%,which is a great enhancement in InGaN-based PEC.